1. Field of Invention
The present invention relates to a process for phosphating metal surfaces, and especially to a process for preparing zinc phosphate coatings containing manganese and magnesium on steel, zinc, aluminum and/or the alloys thereof. These zinc phosphate layers containing manganese and magnesium are applied by spraying, spray-immersion and immersion with aqueous solutions.
2. Statement of Related Art
Processes for phosphating surfaces of iron, steel, zinc and the alloys thereof as well as of aluminum since long have been state of the art (Ullmanns Encyklopadie der technischen Chemie, 4th Edition, Volume 15, pages 686 and 687). Phosphating said surfaces serves to increase the adhesion strength of paint layers and to improve the protection from corrosion.
Of the greatest importance as phosphating processes are acidic solutions of zinc and alkali phosphates. For example, zinc phosphating baths may contain monozinc phosphate, free phosphoric acid, zinc nitrate and oxidizing agents as the main components. The pH value of such compositions is conventionally in the range between 2.8 and 3.4. The course of the process essentially consists of two reactions: the mordanting reaction and the formation of a zinc phosphate layer on the surface to be phosphated.
From W. A. Roland and K. -H. Gottwald, "Metalloberflache", 42nd Year 1988/6 there have been known manganese-modified zinc phosphate coatings as adhesion primer for modern paint coatings. Here it is set forth that the use of manganese ions besides zinc and nickel ions in low-zinc phosphating processes, especially upon use of surface-modified thin sheets, demonstrably improves the anticorrosive property. The incorporation of manganese in the zinc phosphate coatings results in smaller and more compact crystals having an increased alkali resistance. At the same time the working range of phosphating baths is extended; aluminum also can be phosphated in a composite with steel and steel which has been galvanized electrolytically or by melt immersion to form a layer, with the quality standard reached in general being ensured.
From EP-A-0 261 704 there has been known a process for producing phosphate coatings on surfaces which are formed of aluminum or alloys thereof as well as at least one of the materials steel or galvanized steel; therefor, in order to achieve the formation of uniform phosphate layers with a high degree of covering power, spraying or spray-immersion is employed using a phosphating solution which may contain, in addition to zinc, phosphate and fluoride, also further cations from the group of nickel, manganese, magnesium and calcium.
From WO 85/03089 there has been known a high-nickel zinc phosphating process. Herein, extremely high nickel concentrations are employed for phosphating. It is generally referred to that part of the nickel may basically be replaced by a series of monovalent or divalent cations. They have been selected, for example, from cobalt, manganese and magnesium. It is further set forth that the nickel content of the solution to be employed must be at least 1.0 g/l. The ratio to be employed between low zinc and high nickel contents is an essential constituent of the technical teaching.